This paper reports results of a design experiment regarding young children's concepts of mass measurement. The research built on an earlier study in which a framework of "growth points" in early mathematics learning and a related, task-based, one-to-one interview to assess children's understanding of the measurement of mass were developed. Prompted by the results and recommendations from the earlier study, five lessons were developed that offered rich learning experiences regarding concepts of mass. The 119 Year 1 and 2 children participating in the study were interviewed using the same protocol before and after the teaching period. The assessment data showed that the majority of these children moved from using nonstandard units to using standard units and instruments for measuring mass. The findings from this study challenge the traditional approach of using informal units for an extended period before the introduction of standard units.